Hitherto, the beam scanning optical apparatus has been used image writing in electrophotographic process, and installed in laser beam printer, laser facsimile device and other output devices of computer, facsimile apparatus, and others. Recently, higher speed and higher resolution are required, and there is a mounting demand for multi-beam scanning optical apparatus using plural beams in order to enhance the speed and resolution further.
A prior art of multi-beam scanning optical apparatus for scanning multiple beams is described.
First, the constitution is explained.
As shown in FIG. 12(a), it comprises a drive circuit 23 for emitting a beam 21 by driving a first light source 1a, and a drive circuit 24 for emitting a beam 22 by driving a second light source 1b. As the beam 21 from the first light source 1a and the beam 22 from the second light source 1b are emitted, an electrostatic latent image is formed on a scanning surface 18a of a uniformly charged photosensitive drum 18. A collimator lens 2a is provided in correspondence to the first light source 1a, and a collimator lens 2b, to the second light source 1b, and the beams 21, 22 from the light sources 1a, 1b are transformed into coherent beams by these collimator lenses 2a, 2b.
On optical paths of the beams 21, 22 passing through the collimator lenses 2a, 2b, prisms 39a, 39b for adjusting the pitch of the beams 21, 22 in the sub-scanning direction, and a beam splitter 4 for adjusting the optical axes of the beams 21, 22 are disposed. Corresponding to these prisms 39a, 39b, respectively, there are optical path correcting means 40a, 40b for adjusting the beam pitch to a specified interval by rotating the prisms 39a, 39b and varying the beam positions of the beams 21, 22 in the sub-scanning direction.
Moreover, on optical paths of the beams 21, 22 from the beam splitter 4 to the photosensitive drum 18, there are disposed a cylindrical lens 41 for focusing the beams 21, 22 in the sub-scanning direction, a deflector 5 having a deflecting surface 6 near the focusing point of the cylindrical lens 41 for deflecting the two beams 21, 22 simultaneously, a scanning lens system 8 for focusing the beams 21, 22 deflected by the deflector 5 on the scanning surface 18a of the photosensitive drum 18, and a mirror 26 for leading the beams 21, 22 up to the scanning surface 18a.
At a position on the optical path of the beams 21, 22 emitted from the beam splitter 4 in a different direction from the deflecting surface 6, a sensor unit 10 is disposed for detecting the pitch interval of the beams 21, 22 in the sub-scanning direction. A magnified view of this sensor unit 10 as seen from the incident direction of the beam is shown in FIG. 12(b), and it is composed of a two-division sensor 35 for detecting the position of the beam 21, and a two-division sensor 36 for detecting the position of the beam 22. The beams 21, 22 are adjusted in position by the prisms 39a, 39b so as to be emitted to the boundary of the corresponding two-division sensors, that is, between a detecting surface 27 and a detecting surface 28 of the two-division sensor 35, and between a detecting surface 29 and a detecting surface 30 of the two-division sensor 36. The two-division sensors 35 and 36 are fixed as being mutually deviated in the sub-scanning direction so as to form a specified beam pitch on the scanning surface 18a.
To control scan timing of the beams 21, 22 scanned by the deflector 5 in the main scanning direction, a synchronism detector 9 is disposed. By this synchronism detector 9, the beams 21, 22 modulated with the intensity corresponding to the image data are emitted by the drive circuits 23, 24 of the light sources 1a, 1b at the timing of specified time. Moreover, a control circuit 17 is provided to rotate the prism 39a on the basis of the signal from the sensor unit 10, and control the beam position of the beam 21 in the sub-scanning direction. Other members than the photosensitive drum 18 are accommodated in a housing 16.
In thus structured multi-beam scanning optical apparatus, the operation is described below, mainly relating to the operation of pitch adjustment of two beams.
First, in order to obtain a favorable image, it is important to maintain the beam pitch always in normal state, which is described while referring to FIGS. 13(a) and 13(b).
FIG. 13(a) shows an image when the beam pitch is adjusted to a specified value, and FIG. 13(b) shows an image when not adjusted to a specified value. When the beam pitch is not adjusted correctly as shown in FIG. 13(b), the beam scanning line interval includes narrow portions and wide portions, and the image is deteriorated extremely. When assembling such multi-beam scanning optical apparatus, the beam pitch must be fixed after adjusting to a specified value, but actually it is hard to keep the beam pitch precisely for a long period due to aging and environmental changes.
Thus, in the multi-beam scanning optical apparatus, it is an important matter to keep the interval of two beams 21, 22 scanned simultaneously at tens of microns in the sub-scanning directions, and this scanning apparatus includes means for achieving this. This is because it is required to correct the beam pitch after assembling, if the two beams 21, 22 are adjusted to a specified beam pitch at the time of assembling the optical system, as the beam pitch is likely to deviate due to aging effects, deformation or distortion of the unit after installation of the apparatus.
The beams 21 and 22 emitted from the first light source 1a and second light source 1b in FIG. 12(a) are adjusted to coherent beams by the collimator lenses 2a and 2b. Consequently, the beams 21 and 22 are changed in their optical path by the prisms 39a, 39b, respectively, and the pitch in the sub-scanning direction is adjusted to a specified interval.
The beams 21, 22 passing through the prisms 39a, 39b are adjusted of their optical axes by the beam splitter 4, and focused in the sub-scanning direction by the cylindrical lens 41. Under the condition of keeping an optical conjugate relation between the deflecting surface 6 and scanning surface 18a, such focusing is effective to alleviate the effect of surface tilting which is slightly inclined in each deflecting surface 6 of a deflector 5. Since the deflecting surface 6 and the scanning surface 18a are in a conjugate relation in the sub-scanning direction, the beam pitch on the deflecting surface 6 are projected on the scanning surface 18a at a multiplying factor of the scanning lens system 8 in the sub-scanning direction. Therefore, the beam pitch can be controlled on the deflecting surface 6, and the beam pitch on the scanning surface 18a can be maintained in a correct state.
Positioning of the beams 21 and 22 in the sub-scanning direction is described below.
When positioning the beam 21 by using the two-division sensor of the sensor unit 10 as shown in FIG. 12(b), first, only the first light source 1a is driven by the drive circuit 23, and the beam 21 is emitted to the two-division sensor 35. At this time, the control circuit 17 adjusts the optical path of the beam 21 in the sub-scanning direction by rotating the prism 39a so that the output to the quantity of light of the detecting surface 27 and the output to the quantity of light of the detecting surface 28 may be at a same level.
Herein, FIGS. 14(a), 14(b) and 14(c) show the relation between the beam position and the output from the detecting surface in adjustment of beam position by using the two-division sensor.
As shown in FIG. 14(a), when the beam 21 is deviated to the detecting surface 27 side, the output level from the detecting surface 27 is higher than that from the detecting surface 28, and the output difference between the detecting surface 28 and the detecting surface 27 based on the detecting surface 28 appears as a positive value. To the contrary, as shown in FIG. 14(c), when the beam 21 is deviated to the detecting surface 28 side, the output level of the detecting surface 28 is higher than that of the detecting surface 27, and the output difference between the detecting surface 28 and the detecting surface 27 based on the detecting surface 28 appears as a negative value. Therefore, moving direction from the present beam position moving direction can be judged from the output level difference between the detecting surfaces 27, 28, so that the prism 39a is rotated while controlling the optical path correcting means 40a by the control circuit 17.
Change of optical path by the prism 39a is achieved by shifting the angle of the prism 39a in the sub-scanning direction, and changing the position of the beam 21 in the sub-scanning direction on the deflecting surface 6. Accordingly, as shown in FIG. 14(b), when the output levels of the detecting surface 27 and detecting surface 28 become same, rotation of the prism 39a is stopped. At this time, the center of the beam 21 is adjusted to the boundary of the detecting surfaces 27, 28. Thus, by rotating the prism 39a, the position of the beam 21 from the first light source 1a can be corrected to a specified position.
Similarly, the beam 22 is adjusted to the boundary of the detecting surfaces 29, 30 of the two-division sensor 36 by the same operation as in optical path correction of the beam 21.
Since the two-division sensor 35 having the detecting surfaces 27, 28 and the two-division sensor 36 having the detecting surfaces 29, 30 are fixed in a state being deviated in the sub-scanning direction by a specified extent (the pitch extent on the scanning surface 18a), by adjusting each beam to the boundary of each two-division sensor as described above, the two beams 21, 22 scanning the scanning surface 18a are controlled to an adequate interval.
Such optical path correction for adjusting the beam pitch of the beams 21, 22 is conducted before the beams 21, 22 are emitted to form an image on the scanning surface 18a. For example, when this beam scanning apparatus is used in writing of laser printer, said correction is conducted within the stabilizing time of rotation of the deflector 5 after input of print start signal.
After optical path correction of the two beams 21, 22, when the beam pitch is adjusted to a specified interval, beam emission corresponding to the image data on the scanning surface 18a is started. Meanwhile, as mentioned above, following the input of print start signal, after driving of the deflector 5 is stabilized and the conditions are prepared for other units about image forming (for example, OPC unit, developer, fixing device), beam emission corresponding to the image data on the scanning surface 18a is started.
Thus, according to the multi-beam scanning optical apparatus, since the image is formed on the scanning surface 18a by two beams 21, 22, beam scanning of higher speed or higher resolution is realized as compared with scanning of one beam.
In such conventional construction, however, in order to adjust the beam pitch, it requires two optical path correcting means 40a, 40b in the optical paths of the beams 21, 22, and the cost of the apparatus is increased.
It also requires a mounting space for the optical path correcting means 40a, 40b, and the degree of freedom of layout design of optical system is limited.
Still more, in order to adjust the beam pitch, it requires multiple sensors, and the cost of the apparatus is also increased in this respect.